An awful lot of people at large in the world began their lives in a dish. Just over 30 years ago, a British baby named Louise Brown became the first viable child conceived by in vitro fertilization. Now the 3.5 million people who have followed her match the population of Lithuania. But bringing those millions into existence was not easy. On average, only a quarter of all IVF attempts with fresh eggs yield a live birth; frozen eggs perform even worse, topping out at just 17%. According to an announcement made yesterday by a team of researchers in the United Kingdom, however, all that may change. There is now a quick and reliable way to check the viability of eggs before fertilization and implantation begin  and two months from now, a previously childless 41-year-old woman will give birth to the baby who proves it. (Read TIME's 1978 article, "The First Test-Tube Baby.")

The biggest reason IVF treatments so often fail is that even in the best of circumstances, not all eggs are created equal. Up to half the eggs younger women produce carry chromosomal abnormalities that make a full-term pregnancy impossible; that number climbs to three-quarters as women age. The problem is, you can't check the health of the chromosomes without damaging the egg and making it useless for conception. What's needed is a way to make a copy of those chromosomes and subject them to analysis, leaving the egg unharmed. The good news is, such a copy already exists. (See the top 10 medical breakthroughs of 2008.)

Healthy human beings carry 46 chromosomes, 23 contributed by the egg and 23 by the sperm. Eggs begin their development process with the full complement of 46, but shed half of them, tucking them into a small genetic bundle known as a polar body, which mirrors the chromosomes of the egg. Investigators at the U.K.'s CARE Fertility clinic, under the direction of Dr. Simon Fischel, harvested nine eggs from a woman who had undergone 13 failed cycles of IVF implantation and, in addition, suffered two miscarriages and an ectopic pregnancy. Using a laser scalpel, they extracted the polar bodies from the eggs and analyzed them. Two of the nine proved defect-free, and both were implanted. One took hold. (Read "Calculating the Odds of a Baby Through IVF.")

"This woman is now in her third trimester and had never before been this far," Fischel told the news conference at which he announced the breakthrough.

So this is a good thing, right? In a lot of ways, yes. Not all women must go through 13 IVF trials before they succeed, but most women undergo multiple attempts, each of which can cost $5,000. Fischel's procedure runs an extra $2,750 or so  but paying a premium price once is easily cheaper than paying the lower price over and over. What's more, while couples who choose the in vitro route dearly want a child, they don't always want more than one. But the high failure rate of the procedure requires that multiple embryos be implanted at once. The result: either no children or a small litter of two, three, four or more.

What spooks many people is the question, Once you start checking eggs for viability, what else can you screen for? Whole cells can already be harvested from embryos, and free-floating embryonic cells can be extracted from the womb throughout a pregnancy. Both procedures would, in theory, allow embryos to be discarded or pregnancies to be terminated for a matter as trivial as height or eye color. But both also allow parents and doctors to decide against bringing to term a baby that would enter the world with a grave and degenerative disease. The ethical conundrum lies in drawing the line. Plenty of parents abort babies found to be carrying the genetic signature of Down syndrome. Other parents of well-loved Down children would dispute that decision.

Fischel's procedure does not give as complete a look at a potential child, since it relies on only half the chromosomes that make up that profile. But it comes close enough that the work as a whole is being overseen by the U.K.'s Human Fertilization and Embryo Authority, a government watchdog group. The agency is generally supportive of the procedure and indeed has been looking for a way to reduce the incidence of multiple births among IVF families. Nonetheless, the fact that there is a moral cop on the beat gives some comfort to the procedure's critics. For the unnamed 41-year-old, little of this matters. In two months, she'll be changing diapers.